Temporal Relationships Between Excitation and Contraction
Any single muscle twitch proceeds through several successive stages, each closely tied to changes in the cell membrane's excitability and the generation of an action potential (AP).
- Latent period (LP) — the time elapsed from the application of the stimulus to the onset of the visible mechanical response. During this stage, the action potential develops and processes of excitation-contraction coupling take place. At this moment, the membrane is in a state of absolute refractory period (complete inexcitability). Sodium channels are inactivated, so a repeated stimulus cannot trigger a new AP or contraction.
- Shortening period (SP) — the phase of direct force generation, or muscle systole. During this time, refractoriness is replaced by the phase of exaltation — a period of heightened (supernormal) excitability.
- Relaxation period (RP) — the phase when the muscle fiber returns to its initial length (muscle diastole).
Mechanisms of Single Twitch Summation
Summation occurs only with repeated stimulation. If the second stimulus arrives after the cycle has fully completed (following the relaxation phase), the muscle successfully returns to its baseline state. In this case, two isolated single contractions of equal amplitude occur, and no summation takes place.
The overlap effect directly depends on the specific phase of the first response in which the repeated impulse arrives:
- Arrival during the relaxation phase: the ongoing relaxation process is immediately interrupted and replaced by a new contraction cycle. The resulting amplitude is greater than the initial one.
- Arrival during the shortening period: the current shortening is prolonged, and the overall amplitude of the muscle response increases.
- Arrival during the latent period: a new contraction does not occur, because the second stimulus coincides with the absolute refractory period of the first excitation.
Development of Tetanic Contraction
If the stimulation frequency increases, single responses begin to fuse into continuous muscle tension. Such fused contraction is called tetanus.
Depending on the stimulation frequency, two main types of tetanus are distinguished:
- Unfused (incomplete) tetanus. Formed when each subsequent impulse arrives during the relaxation period of the preceding single cycle. The new tension is superimposed on the previous one before the muscle has fully relaxed. Graphical recording reveals distinct peaks ("teeth"), but the baseline tension level significantly exceeds that of a single twitch.
- Fused (complete) tetanus. Develops with a further increase in frequency, when each subsequent stimulus arrives strictly during the shortening period of the preceding response. Contractions completely fuse without the slightest sign of relaxation. As a result, a uniform, powerful, and sustained tension is formed, the amplitude of which is several times greater than a single twitch.
Frequency Optimum and Pessimum
The amplitude of fused muscle tension cannot grow indefinitely. It strictly depends on the ratio between the stimulation frequency and the tissue's lability.
Optimum — the stimulation frequency at which the maximum amplitude of tetanic contraction is recorded. The main condition for the optimum: each subsequent impulse must fall not only within the shortening period, but precisely into the exaltation phase (heightened excitability) left over from the previous cycle.
Pessimum — a sharp drop in amplitude (muscle relaxation) during rhythmic stimulation at a frequency exceeding the tissue's lability. This phenomenon is also known as Vvedensky inhibition. The condition for pessimum: the frequency is so high that every new impulse falls into the latent period and coincides with the refractory period (inexcitability) of the preceding action potential. The tissue is physically incapable of responding to the stimulus.